The BBC has put up an interactive web page with the parameters of the Drake equation that lets you tweak numbers and estimate how many alien civilizations might exist. It’s informative because you should quickly realize you can make up any old numbers you want for most of them — we simply don’t have data for most of them, so you have to reach up into your colon to pluck something random out.
They have various presets, including a modern “skeptical estimate”. I just looked at the section on life, and found it weirdly inconsistent. Apparently, the % chance a habitable planet develops life is guessed at 13% (I don’t buy it; that life arose so quickly on Earth after its formation suggests that it may be relatively easy — I’d jack that up to something high), while the % chance that life develops intelligence is pegged at an absurd 50%. We’ve got one planet with tens of millions of species for a data point, and our kind of intelligence popped up once in 4 billion years. It makes no sense to argue for that degree of inevitability for a weird and unlikely adaptation like intelligence…why didn’t it arise in the Mesozoic, then?
Their worst estimate (which looks ridiculously optimistic to me) ends up with only about one civilization per galaxy. That’s also with a conservative estimate that a civilization only spends about 400 years trying to send signals outwards…which would mean that a brief effort to talk to some other civilization within a disc 100,000 light years in diameter is almost certainly doomed to failure. Even in their most optimistic model, with tens of thousands of technological civilizations in a galaxy, stars populated by intelligent life are still about 600 light years apart.
At least the web page makes it obvious that the Drake Equation is like a Ouija board, with the tinkerers just pushing the numbers around until they get the answer they want.